ES2238669T5 - HOT LAMINATED STEEL BAND AND PROCEDURE FOR MANUFACTURING. - Google Patents

Abstract

A hot-rolled steel strip rolled to a final thickness that is at least 2 mm but no more than 12 mm, with a microstructure comprising at least 95 % martensite and/or bainite. The steel contains, in percentages by weight: 0.08 % - 0.16 % C, 0.5 % - 1.5% Cr and/or 0.1 % - 0.5 % Mo, ≤ 0.015 % S and ≤ 0.03 % P, 0.01 % - 0.08 % Al, as well as 0.6 % - 1.1 % Mn, and 0.1 % - 0.3 % Si, the rest being Fe and unavoidable impurities. The tensile strength of the steel strip is 700 Mpa - 1500 Mpa at a tensile elongation that has an A5 value of at least 6 %, and the yield strength is 600 Mpa - 1400 Mpa.

Description

The invention relates to a steel strip that is hot rolled to a final thickness of at least 2 mm but not more than 12 mm, in which the microstructure of said steel strip comprises at least 95% martensite and / or bainite and in which the steel contains, in percentages by weight: 0.08% - 0.16% of C, 0.5% - 1.5% of Cr and / or 0.1% - 0, 5% of Mo, ≤ 0.015% of S and ≤ 0.03% of P, 0.01% - 0.08% of Al, and the rest is Fe and inevitable impurities. The invention also relates to a process for manufacturing said hot rolled steel strip.

Traditionally, hard steels have been made by annealing and cooling, but this technique has not been achieved, for example the optimal surface quality and impact hardness. Manufacturing costs have also been high.

In GB-2 195 658, a floor steel is described, containing in a preferred embodiment 0.05% - 0.08% carbon, 0.1% - 0.5% silicon, 0.5 % -1.6% manganese, 0.5% - 1.5% chromium, up to 0.05% titanium, up to 0.1% niobium, 0.005% - 0.012% nitrogen, up to 0 , 06% aluminum and 0.002% 0.005% boron. Furthermore, according to said publication, the floor is started at a temperature of 1,200 ° C - 1,275 ° C, and the forged object is cooled in a bath, so that the temperature of the object is measured continuously, and the cooling is interrupted before it ends. The transformation into martensite. In this way the tensile strength of 700-1,100 N / mm2 is obtained, and at the same time a satisfactory impact resistance is obtained in addition to a PS / TS ratio of approximately 0.75 without tempering separately or other heat treatment.

In contrast to said steels used for forging, the known resistant steel bands, that is, the steels used in rolling, have a high manganese content and often also a fairly high carbon content, such as the rolled steel strip in hot described in publication US-6 284 063 having a thickness not exceeding 5 mm. The steel described in said publication contains, in percentages by weight, 0.08% - 0.25% carbon, 1.2% - 2.0% manganese, 0.02% - 0.05% aluminum and less 0.07% silicon, plus up to 0.015% phosphorus and up to 0.003% sulfur, while the hot band contains more than 95% martensite. Steel can also contain up to 1.0% chromium, up to 0.1% copper, up to 0.5% molybdenum, up to 0.1% nickel, up to 0.009% nitrogen, up to 0.0025% boron and possibly titanium in a stoichiometric part, Ti = 3.4x% N, with respect to the amount of nitrogen. First, the billet is heated to a temperature of 1,000 ° C - 1,300 ° C, pre-laminated within the temperature range of 950 ° C 1,150 ° C and finalized at a final lamination temperature above Ar3. The hot band produced in this way is cooled to a winding temperature in the range of 20 ° C below the initial temperature of the MS martensite, so that the content of the other forms of the phase except that of martensite is of Less than 5%. According to said publication, cooling to the winding temperature is preferably carried out so that the cooling time in the range of 800 ° C → 500 ° C is less than 10 seconds. In this way, a tensile strength is obtained for the final product that is in the range of 800N / mm2 -1,400N / mm2.

Publication US-4 406 713 represents a process for making high strength, high strength steel with good moldability and weldability, said steel containing 0.005% - 0.3% carbon, 0.3% - 2.5% manganese , up to 1.5% silicon, up to 0.1% niobium, up to 0.15% vanadium, up to 0.3% titanium and up to 0.3% zirconium. According to the procedure, the austenization is carried out at a temperature of 1,000 ° C - 1,300 ° C, and thereafter first, for example, hot rolling is carried out in the temperature range of Ar3-930 ° C, when the recrystallization of the austenite is has significantly delayed, in an area reduction of at least 30%. This kind of work introduces a lot of rigidity in austenite, which displaces the precipitation temperature range of the ferrite phase in a usual CCT diagram at higher temperatures and shorter times. In the course of refrigeration after work, carbon is concentrated in the unprocessed austenite phase as precipitation of the ferrite phase proceeds. After the ferrite has occupied 5 - 65% of the steel, the steel cools rapidly below the temperature Ms, and a two-phase structure comprising fine grains of ferrite and martensite with a high temperature can be obtained in the steel carbon concentration

GB-2 076 425 describes a process for producing dual phase steel, a process in which the steel strip is hot rolled, hot rolled is finished at approximately 900 ° C, and wound at a temperature between about 350 ° C and approximately 580 ° C, and in which the band is subsequently continuously coated in the zone of the two phases austenite ferrite at temperatures between 760 ° C and 830 ° C with a maintenance time between 1.5 and 3 minutes followed by cooling with a speed of 3.5 at 6 ° C / s to transform at least most of the austenite to martensite. The composition of the steel comprises, by weight, from 0.03% to 0.25% carbon, from 0.3% to 2.5% manganese, up to 1.5% silicon, up to 0.25% of molybdenum and up to 2% chromium, the remainder being iron except additional and residual impurities in amounts that depend on the practice of metallurgy. The hot rolled strip is cold rolled subsequently before the mentioned annealing at temperatures between 760 ° C and 830 ° C. The latter annealing is terminated by forced or natural air cooling.

image 1

Patent publication US 6,554,919 (the published application was US 2001/0049956) describes, according to its claim 1, a hot rolled steel with a very high maximum elasticity and mechanical strength that can be used in particular to produce parts of cars, said steel comprising a completely bainitic structure, and the following composition by weight: 0.08% <carbon <0.2%, 1% <manganese <2%, 0.02% <aluminum <0.1%, silicon <0.5%, phosphorus <0.03%, sulfur <0.01%, 0.1% <vanadium <0.3%, chromium <1%, nitrogen <0.015%, molybdenum <0.6%, the rest being steel and impurities inherent to the processing, in which said steel does not comprise niobium. The patent publication also describes a process for producing a strip of hot rolled steel sheet, in which said alloy is subjected to: rolling at a temperature below 950 ° C, cooling carried out at a rate greater than 20 ° C per second until a temperature in the range of 400 ° C to 600 ° C. US 6,554,919 uses pure bainitic structure teaching that martensitic steels actually have the highest levels of resistance, but that it is difficult to produce such a structure in a broadband train due to the fragility of the martensite, which makes the band it breaks after rolling, and therefore the martensitic steels can reach resistance levels greater than 1,000 MPa but with very low ductility levels and dilations less than 8%. US 6,554,919 further describes that a heat treatment must be carried out after rolling, because the martensite structure is to be obtained by heat treatment after rolling. US 6,554,919 also teaches that: "Vanadium increases mechanical strength and reduces dilation. Vanadium is the necessary element in steel with a bainitic structure in order to produce a hardening effect, something that was not expected since the microalloy elements have a precipitation hardening effect, but this precipitation ends at a more temperature high and should be carried out at the site of the ferrite to harden. This effect cannot be obtained by other micro-alloy elements such as titanium or niobium, because these elements produce an increase in hot hardness, thus limiting the reduction of hot rolling speeds and therefore the minimum thickness that It can be obtained for this kind of sheet metal. It turns out that vanadium has no effect on hot hardness. Other residual elements may be present and used according to their known properties, such as Cu and Ni. Added alloy elements, such as titanium or boron, can be used to promote precipitation of vanadium carbides at the expense of vanadium nitrides. Titanium and boron form nitrides at high temperature, which remain stable during the subsequent heat treatment. ”

The objective of the present invention is to achieve a hot rolled steel strip and its manufacturing process such that the steel is not critical in regard to local fluctuations of the winding temperature in the strip, which would be highly weldable, suitable for cutting and thermal bending and with high strength and, particularly, high impact resistance. Another object of the invention is to perform this kind of hot rolled steel strip and its manufacturing process that would allow for economical production costs.

The invention is reflected in the claims.

According to a first principle of the invention, the first defined hot rolled steel strip also contains 0.6% - 1.1% Mn and 0.1% - 0.3% Si; The tensile strength of the steel band is 700 Mpa - 1,500Mpa with a tensile elongation that has an A5 value that is at least 6%, and the elastic limit is 600 Mpa

1,400 Mpa. According to another principle of the invention, this kind of steel strip is manufactured by a process comprising the following steps: hot rolling of the steel strip in the temperature range of 860 ° C - 960 ° C up to said final thickness; direct cooling of said hot rolled steel strip with a delay of no more than 15 seconds from the last rolling pass at the winding temperature within the range of 100 ° C - 520 ° C, so that the cooling rate in direct cooling It is at least 30ºC / s. No annealing is performed by tempering.

The innovative idea is based on the fact that by reducing the amount of manganese and carbon, as well as by the alloy of chromium and / or molybdenum, in addition to boron when necessary, good hardness can be maintained and the following advantages are achieved. The steel structure is not critical for the segregation of manganese and carbon during the smelting process due to the low content of manganese and carbon. The properties of steel are not critical for local fluctuations in the winding temperature in the band, which facilitates the production of steel and has an advantageous effect on the homogeneity of its mechanical properties, which again has a positive influence on both the flatness of the final product as in the residual effort. The steel plate is very suitable for welding and laser cutting, and at the same time has good fatigue resistance regardless of such heat treatments. In addition, the steel plate has excellent bending properties, good impact resistance and good resistance to softening when tempering.

By manufacturing this type of steel instead of the traditional oven annealing and cooling, by cooling directly after hot rolling, excellent impact resistance is achieved, because the phase transformation into martensite and / or bainite takes place from an austenite of fine and worked grain. In the same way the surface quality is improved, since the main inlays are removed in a descaler before rolling. Manufacturing costs are also reduced along with the rationalization of the procedure. In a strip lamination line, a high heating temperature is typically applied in the oven, for example in the range of 1000 ° C - 1300 ° C, and a long maintenance time, for example from 2h - 10h. In that case the dissolution of special carbides, such as Cr and Mo carbides, and the homogenization of the structure are as complete as possible. On the other hand, the growth of the austenite grain at the high heating temperature does not make the final product more fragile, because the austenite is finely granulated during hot rolling. In addition, excellent hardness is achieved, combined with excellent impact resistance.

image2

The hot rolled steel strip according to the invention that is hot rolled directly to a thickness of 2mm-12mm can be made wear resistant and with different hardnesses, typically in the hardness range of 300 HB-400 HB, such as the so-called plates of wear-resistant steel with the same production procedure of the structural steel plates, changing only the analysis and / or the speed of post-lamination cooling of the web, and / or the temperature before winding, within the scope of the invention. This class of wear-resistant steel can also be used in cases where the structures require properties typically demanded by structural steel, such as good moldability, weldability and impact resistance, which means that the hot rolled steel strip according to the invention is feasible also as structural steel. In the analysis of the steel that will be explained in the following description, all the percentages of content are percentages by weight, and the rest of the steel not otherwise defined is iron, Fe, and unavoidable impurities.

First, the steel according to the invention has a relatively low carbon content, that is at least 0.08% C but not more than 0.16% C for good impact resistance, flexibility and weldability. The phosphorus P it contains as an impurity can increase up to 0.03%, and the sulfur S can respectively increase up to 0.015%, which means that these contents are limited to achieve good impact resistance and flexibility. When necessary, other properties can be improved by treating the laundry with Ca or CaSi. The deoxidizing agent used is aluminum, which in the final product can represent at least 0.01% of Al but not more than 0.08% of Al. Chromium, at least 0.5% of Cr but not more than 1.5% Cr, and / or molybdenum that is at least 0.1% Mo but not more than 0.5% Mo, is alloyed to increase hardening and temper resistance. This allows precipitation at higher winding temperatures, which can be used for the reduction and even the prevention of softening of the steel, as well as to alleviate the voltage fluctuations caused by local temperature differences during the cooling of the coil.

As for other high-strength steel bands of the same type, the manganese content is at least only 0.6% Mn but not more than only 1.1% Mn. Therefore, steel is not susceptible to the segregation of manganese and carbon, which improves the homogeneity of the microstructure. In the tests that were carried out it was observed that this is the way to achieve good flexibility properties and even mechanical properties in different directions, as well as a high surface quality when thermally cut. As for silicon, it serves as a deoxidizing agent in the steel of the present invention, and also functions as a solid hardener solution in area contents of at least 0.10% Si and up to 0.30% Si, which It has an advantageous effect on impact resistance and moldability.

The steel according to the invention can be thermally cut, for example by laser, into precisely defined figures. It has been observed that an extraordinarily smooth cut surface is achieved in a laser cut object. On the other hand, it has been found that the difference in strength between the basic material and the soft zone created in the technical cutting process, an area that is located near the hardened area, is relatively small. All this has an advantageous effect on fatigue resistance. In addition, a low carbon content reduces the maximum hardness of the hardened area, so that the cutting surface is not sensitive to brittleness and cracking, neither working the object nor in practical use.

According to the test analyzes presented here, the copper contents were not remarkable, but according to other tests not illustrated here, it can still be maintained that the copper content must be limited to less than 0.3% Cu to ensure quality Excellent surface of hot rolled strip. If the copper content exceeds 0.3%, it is advisable to also allocate the nickel, at least 0.25 times the copper content. Even if there is no copper in the alloy, the amount of nickel is restricted to ≤ 1.5% Ni.

The amount of alloyed boron is typically at least 0.0005% B but not more than 0.005% B to reduce grain size and increase hardening capacity. The amount of alloyed titanium is typically at least 0.01% Ti but not more than 0.1% to bind N nitrogen and prevent the formation of BN boron nitrides, because boron nitride reduces the effectiveness of boron as hardening enhancer and grain size reducer.

The steel according to the invention may, particularly at the lower limit of the carbon content, be well flexed with respect to its strength, that is, for example, welded in a high-frequency weld free of input metal, called HF welding, in a tube . In test production it was also discovered that the material is extremely well suited in the production of both open profiles and the hollow sections of the HF soldier.

According to the invention, the steel is manufactured at a final rolling temperature that is within the range of 860 ° C - 960 ° C, up to a final thickness of 2mm - 12mm. The cooling of the web begins no later than 15 seconds after the last rolling pass, and cools quickly, the cooling speed being at

image3

5

10

fifteen

twenty

25

30

35

less than 30ºC / s, up to a low winding temperature in the range of 100ºC - 520ºC. The result obtained is typically a microstructure close to being completely bainitic and / or martensitic, so that the content of bainite and / or martensite is at least 95% by volume. In the winding temperature range of 20 ° C - 100 ° C, the martensite would not be tempered, while when the winding temperature is at least 100 ° C, the martensite is tempered, so that for example in the range of 100 ° C - 200 ° C, the Martensite is slightly tempered, and in the approximate winding temperature range of 200ºC - 520ºC, the martensite is tempered and the carbon precipitates. Although winding was carried out in a range of brittleness at the lowest tempering, 200 ° C 400 ° C, or cooling was carried out in said range, no brittleness was observed when tuning with the combination of this production process and the composition. The tensile strength obtained Rm is approximately 700 Mpa - 1500 Mpa, and the elastic limit obtained Rp0.2, ie resistance to an elongation of 0.2%, is approximately 600 Mpa - 1,400 Mpa. The resistance to elongation A5 is consequently approximately 18% -6%. The Y / T transfer rate is typically in the range of 0.8-0.96.

When it is desired to obtain hard surface plates, particularly resistant to wear, the carbon content of the steel can be arranged in the range of 0.12% - 0.16% C, and the hot rolled steel strip can in that case cool directly to the winding temperature, which is above 100 ° C, but below 400 ° C, in which case the residual stress is reduced or disappears, however, without affecting the hardness of the wear plate. Thus, a relatively low winding temperature, in the range of 100 ° C 200 ° C, can be applied for example for thinner bands, or a slightly higher winding temperature, in the range of 200 ° C - 400 ° C, for example for thicker bands . If, on the other hand, more properties of the structural steel type are desired, the carbon content of the steel is arranged in the range of 0.08% 0.12% C, and the hot rolled steel strip is cooled directly at the winding temperature, which is above 100 ° C, but below 520 ° C. For example, a relatively low winding temperature, in the range of 100 ° C - 200 ° C, can be applied to thinner bands, and for example a slightly higher winding temperature, in the range of 200 ° C - 520 ° C, can be applied to thicker bands. In this case of "structural steel", that is to say with a carbon content in the range of 0.08% - 0.12%, the fluctuations of the winding temperature of the order described above have, however, a really restrictive effect. of the properties of the steel strip, when it remains in good condition regardless of the winding temperature.

Examples

Example 1. Traditional tempering tests were carried out in a laboratory with composition a1, see table 1, by heating samples with measurements of 8x100x250 mm, in an oven for 20 minutes and at a temperature of 900 ° C. The samples were cooled in water and tempered for 2 hours at different temperatures. The results are presented in table 2. The results show that the material has a low resistance area in the temperature range of 250ºC - 350ºC. On the other hand, elongation clearly increases at tempering temperatures above 400 ° C, in which case the resistance also begins to fall.

Table 1. Table2.

Test compositions

C

Yes Mn P S To the N Cr Mo You B

steel A

a1

0.098 0.22 0.71 0.008 0.004 0.030 0.005 0.94 0.20 0.032 0.002

a2

0.086 0.28 0.77 0.008 0.003 0.024 0.005 0.82 0.27 0.032 0.002

a3

0.083 0.21 0.77 0.010 0.003 0.033 0.005 1.04 0.27 0.036 0.002

steel B

b1

0,140 0.26 0.81 0,110 0.003 0.027 0.006 0.65 0.21 0.038 0.002

b2

0.166 0.23 0.82 0.006 0.003 0.032 0.007 0.88 0.27 0.036 0.002

b3

0.135 0.23 0.90 0.009 0.004 0.035 0.006 0.88 0.27 0.038 0.002

b4

0,130 0.25 0.84 0.008 0.002 0.032 0.005 1.06 0.28 0.037 0.002

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Tempering test results with composition a1

Ttemple ºC

Time h Rp0.2 N / mm2 Rm N / mm2 TO 5 % Charpy in V, J / cm2 resistance, %

(-20ºC)

(-40ºC) (-20ºC)  (-40ºC)

*)

972 1072 12.6 twenty 5

100

2 897 1123 11.7 133 85 40 fifteen

150

2 913 1125 12.0 172 72 65 10

200

2 922 1113 12.4 122 fifty 40 10

250

2 938 1112 12.2 36 26 10 10

300

2 928 1086 11.7 55 28 10 5

350

2 963 1064 11.8 115 27 40 10

400

2 971 1049 12.6 93 58 twenty fifteen

450

2 911 960 14.2 218 85 80 fifteen

500

2 822 901 15.1 251 216 98 80

600

2 741 773 17.3 334 329 100 98

700

2 430 528 21.2 430 451 100 100

*) only cooled

Direct cooling tests on the low carbon production scale

5 Example 2. In the laminating line of the web, there was a 6 mm thick hot-rolled web with an a2 composition by direct cooling to TBOBINA winding temperature. The results are presented in table 3.

The results warn that also when the winding is in the temperature range of fragility at tempering of 300 ° C, as shown in example 1, excellent resistance is still achieved. The strength and elongation do not differ much from Example 1. The results of the material flexibility tests are illustrated in Table 4.

Example 3. In the laminating line of the web, there was a 3mm thick hot-rolled web with the a2 composition by direct cooling to TBOBINA winding temperature. The results are presented in table 3.

The results are warned that also when refrigerated at a clearly higher temperature of 450 ° C, the same mechanical properties as in Example 2 were still achieved.

Example 4. In the laminating line of the web, there was a 3mm thick hot-rolled web with the a2 composition by direct cooling to TBOBINA winding temperature. The results are presented in table 3.

The results are warned that also when cooling at a clearly lower temperature, that is to say at 100 ° C, the same mechanical properties of examples 2 and 3 were still achieved.

It can be concluded that by means of this composition and the steel manufacturing process, a homogeneous material is achieved that is not sensitive to fluctuations in the winding temperature.

Example 5. In the laminating line of the web, there was a 10 mm thick hot-rolled web with the

Composition a3 by direct cooling to TBOBINA winding temperature. The results are presented in Table 3. The results show that the resistance and impact hardness are somewhat reduced, but the properties are still excellent, as long as the winding temperature does not exceed approximately 500 ° C.

image5

Table 3.

image6

Mechanical properties of the web as a result of lamination tests

Longitudinal

Cross

steel

 thickness mm width mm TBOBINE ºC Rp0.2 N / mm2 Rm N / mm2 Y / T TO 5 % HB ChV -40ºC J / cm2 Rp0.2 N / mm2 Rm N / mm2 TO 5 %

a1 a1 a2 a2 a2 a2 a3 a3 a3 a3 b1 b2 b2 b2 b3 b3 b4

8 8 3 3 4 6 10 10 10 10 4 4 4 4 4 4 6 1000 1000 1000 1000 1250 1250 1250 1250 1300 1500 1500 1500 1250 1250 1250 a * b * 460 450 100 200 520 510 370 320 470 515 530 100 380 200 200 971 897 958 971 977 934 748 836 853 858 980 860 702 1179 1163 1125 1125 1049 1123 1030 1014 1117 1078 874 901 965 979 1031 1000 853 1347 1275 1317 1295 0.93 0.80 0.93 0.96 0.87 0.87 0.86 0.93 0.88 0.88 0.95 0.86 0.82 0.88 0.91 0.85 0, 87 12.6 11.7 10.9 11.8 13.3 12.8 13.0 13.0 11.5 11.1 10.0 12.4 17.4 8.9 9.6 11.5 9, 5 304 299 329 304 295 252 396 375 387 384 57 25 240 71 133 171 165 925 977 987 920 819 896 898 914 1051 974 747 1189 1162 1130 1016 1056 1130 1070 899 957 975 1005 1071 1006 847 1308 1294 1333 10.5 9.9 11.6 9.9 11.8 11.0 9.5 10.8 8.4 9.9 13.8 6.9 6.8 8.9

a * Traditional laboratory test: austenization, water cooling, tempered at 400ºC, 2h b * Traditional laboratory test: austenization, water cooling, tempered at 100ºC, 2h

Direct cooling tests on the production scale with a high carbon level

Example 6. In the laminating line of the web there was, with a very high level of carbon, a 4 mm thick strip hot rolled with the compositions b2 and b3 by direct cooling to the winding temperature

5 TBOBINE. The winding temperatures applied in the tests were 100 ° C, 200 ° C and 380 ° C. The results are presented in table 3. The results show that the resistance and hardness decrease somewhat with the winding temperature increases, but the properties are still of the same type, as long as the winding temperature does not exceed approximately 400 ° C. .

10 We can conclude that with this steel composition and the manufacturing process, a homogeneous material is achieved that is not sensitive to winding temperature fluctuations.

Example 7. In the laminating line of the web there was, with a high carbon level, a 4 mm thick hot-rolled web with a composition b1 and b2, by direct cooling to the coiling temperature TBOBINA. The winding temperatures applied in the tests were 470 ° C, 515 ° C and 530 ° C. The results are

15 presented in table 3. The results show that resistance and hardness decrease, while elongation clearly increases with the rise in winding temperature.

Table 4

Flexibility tests with composition a2, winding temperature of 300ºC

R =

Longitudinal in the direction of lamination Crosswise in the direction of lamination

3t

okay okay

2.5t

okay okay

2t

okay (ok), slight cracks on the surface

1.5t

okay deep cracks

1t

(ok), slight cracks on the surface deep cracks

0.7t

(ok), slight cracks on the surface

Radius of curvature = R, plate thickness = t

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Claims (5)

 R E I V I N D I C A C I O N E S 1. A steel band with a microstructure comprising martensite and / or bainite, and in which the steel contains, in weight percentages: 0.08% - 0.16% C, 0.5% - 1.5 % Cr and / or 0.1% - 0.5% Mo, 0.6% - 1.1% Mn, ≤ 0.015% S and ≤ 0.03% P, 0.01% - 0 , 08% of Al, 0.1% - 0.3% of Si, 0.0005% - 0.005% of B and 0.01% - 0.1% of Ti, the rest being Fe and impurities unavoidable, being the tensile strength of the 700 Mpa - 1500 Mpa steel band with tensile elongation, whose A5 value is at least 6%; in which the steel strip is a hot rolled steel strip, laminated to a final thickness of at least 2 mm but not more than 12 mm; the microstructure comprises at least 95% martensite and / or bainite; and the elastic limit is 600 Mpa - 1400 Mpa; and said hot rolled steel strip has an elasticity limit in the range of 0.8-0.96. 2. A process for the manufacture of a steel strip having a microstructure comprising at least 95% of martensite and / or bainite, said steel containing weight percentages of: 0.08% - 0.16% C; 0.5% - 1.5% Cr and / or 0.1% - 0.5% Mo; 0.01% - 0.08% Al; 0.6% - 1.1% of Mn; 0.1% - 0.3% of Si, 0.0005% - 0.005% of B, and 0.01% - 0.1% of Ti, in addition to the rest of Fe and unavoidable impurities, with the rolled steel strip being hot in the temperature range of 860 ° C - 960 ° C, in which the process includes the following steps:

-

 said hot rolling in said temperature range provides a final thickness for said steel strip of at least 2 mm but not more than 12 mm;

-

 This hot rolled steel strip cools directly with a delay of no more than 15 seconds since the last rolling pass at a winding temperature in the range of 100ºC - 520ºC, so that the cooling rate in this direct cooling is by at least 30ºC / s.

3.

 A process according to claim 2, characterized in that the carbon content of the steel is arranged in the range of 0.12% - 0.16% C, and said hot rolled steel strip is cooled directly to the winding temperature in the range of 100 ° C - 200 ° C, or in the range of 200 ° C - 400 ° C.

Four.

 A process according to claim 2, characterized in that the carbon content of the steel is arranged in the range of 0.08% - 0.12% C, and said hot rolled steel strip is cooled directly to the winding temperature in the range of 100 ° C-200 ° C, or in the range of 200 ° C - 520 ° C.

5.

 A process according to claim 2, characterized in that no tempering with tempering or any additional cooling in the process is carried out.